Hydrothermal crystal growth processes are utilized for growing a wide variety of single crystals. In a typical hydrothermal process, a pressure vessel, e.g., a cylindrical pressure vessel, having a baffle to produce a temperature gradient is partially filled at room temperature with an aqueous solution (a growth medium). A solid form nutrient material or feedstock (e.g., powder or crystalline) is introduced on one side of the baffle and one or more seed crystals are introduced on the other side of the baffle.
To ensure solubility and phase stability of the nutrient in the growth medium, compositions such as hydroxides, fluorides, phosphates, borates, and chlorides (typically denominated mineralizers) are added to the growth medium. These mineralizers also have an effect of decreasing the pressure encountered at a given temperature while increasing the solubility of the nutrient in the growth medium.
The pressure vessel is sealed and heating is initiated on the nutrient end to establish a thermal gradient thereby creating two thermal zones. As the temperature rises, the growth medium expands, fills the vessel (in some instances), and concomitantly causes an elevation in the internal pressure of the vessel.
At the relatively hotter end, the solid form nutrient dissolves into the growth medium, while at the cooler end it is deposited on a seed crystal, growing the desired crystal.
Various commercially available ZnSe crystals are currently produced by a chemical vapor deposition (“CVD”) technique. However, ZnSe crystals grown by the commercial CVD technique suffers from multiple grains with various orientations when ZnSe crystals of substantial size (greater than about 5 mm2) are fabricated, and therefore are not suitable for use toward producing orientation-patterned (“OP”) ZnSe substrates.
Additionally, previous hydrothermal growth of ZnSe described by Laudise, et al. (see “Hydrothermal Crystallization of Some II-VI Compounds” J. Cryst. Growth. 3,4, 422-425 (1968); and “Hydrothermal Crystallization of Zinc Selenide” J. Cryst. Growth. 7, 199-201 (1970)), detailed growth of ZnSe by spontaneous nucleation and transport growth in NaOH and NaOH/LiOH solutions at temperatures of 350° C. Laudise et al. achieved hydrothermal growth of ZnSe onto a ZnSe seed crystal of CVD origin and therefore demonstrated bulk growth possibilities by hydrothermal synthesis. However, the conditions employed require a very small temperature gradient, ranging from about 2° C. to about 5° C., which is difficult to control and maintain. With the exception of the (111) face, Laudise et al.'s method manifested in the overall growth quality having cracks and inclusions in the grown crystal. Additionally, spurious nucleation in the upper section of the reaction vessel detracted from growth onto the seed crystal.
Thus, there remains a need for new hydrothermal methods of growing high quality zinc selenide crystals of high purity and significant size.